My EQ2 guild does this. We have a chat channel opened and friends (mostly from EQ1 who migrated to EQ2 but on different severs) can chat with us via the channel.
I am pretty sure it is /join server.channel
I have not tried opening a channel on a different server. We open the channel on our server and they chat with us on our channel.
Edited, Thu May 19 17:34:45 2005 by Imake